The purpose of the Contractility Core is to provide collaborating investigators in the Program Project with reliable and valid measures of the contractile properties of whole striated muscles and of strips of diaphragm muscle fibers, statistical analysis of the data, and interpretation of the structure-function relationships. The Core will also provide opportunities for instruction and training for faculty and trainees who wish to learn the techniques. Muscle fibers when activated attempt to contract or shorten. Whether an activated myofiber shortens, stays at the same length or is stretched depends on the interaction between the force developed and the load. For striated muscle, contractility is defined as the capability of muscle fibers to develop force during fixed length or isometric contractions, during shortening or miometric contractions, and during lengthening or pliometric contractions. Our working hypothesis is that contractility of striated muscles is a complex phenomenon and the provision of reliable and valid measurements requires sophisticated equipment and highly trained muscle mechanicists to make the measurements and analyze and interpret the results. Consequently, to test hypothesis relating the underlying mechanisms of the reduced contractility of striated muscles in diseased and old animals rigorously, a Contractility Core is a necessity. Impairments during each of the three types of contraction may occur at any age due to injury or disease or as an intractable concomitant of aging. The impairments in contractility, whether due to injury, disease, or old age, limit the activities of daily living and reduce the quality of life, particularly for the sick and the elderly.